A. Field of the Invention
The present invention relates to ultrasonic bonding machines of the type which use an ultrasonically vibrated tool to bond conductive interconnecting wires to conductive sites on miniature electronic devices such as integrated circuits and magnetic read/write heads used in disk drive memories. More particularly, the invention relates to a method and apparatus for detecting failure of an ultrasonic bonding tool of a bonding machine used to make a sequence of wire interconnections between pairs of bond sites, to bond a wire to one or more of such sites.
B. Description of Background Art
Miniature electronic circuits, or “micro-circuits,” are used in vast quantities, in a wide variety of consumer, commercial, industrial and military devices and equipment. The majority of such micro-circuits are of a type referred to as integrated circuits. Integrated circuits contain a number of active circuit elements such as transistors, and passive elements such as resistors and capacitors mounted on a substrate. Semiconductor integrated circuits consist of a small monolithic chip made of a semiconducting material such as silicon having discrete areas into which impurities are diffused to form circuit elements and having conductive paths between circuit elements on the chip formed by selectively etching metallized layers of the chip. In hybrid micro-circuits, circuit elements mounted on a ceramic substrate are usually interconnected by conductive ink paths on the substrate.
Functional portions of integrated circuits are typically in the form of very small, rectangular-shaped chips, ranging in size from 0.025 inch to 0.200 inch or more on a side. Input connections to integrated circuit chips are often made by bonding a very fine wire to electrically conductive pads on the chips, the other end of each wire being bonded to a conductive terminal that is sufficiently large and robust to be soldered to conductors on a circuit board. Wire bonding of this type utilizes ultrasonic energy and/or heat to form an intermetallic bond or weld between the wire and metallic bond site. Such wire bonds are used to form interconnections between conductive pads of an integrated circuit chip and terminals of a package used to enclose and protect the chip, and are also used to connect lead-out terminals to delicate read/write heads used in disk memories.
Bonding wires used to interconnect the pads of a semiconductor chip to terminals of a package containing the chip are generally made of aluminum or gold, and have a diameter of about 1 mil (0.001 inch) or less. Each interconnecting wire must be bonded to the upper surface of a small, typically rectangular-shaped, integrated circuit pad a few mils wide at one end of the wire to form a first bond site, and to another similarly shaped pad, or to a larger package terminal comprising a second bond site. In some cases, a length of bonding wire is interconnected to three or more pads, in a “daisy-chain” fashion referred to as stitch bonding.
The most common method of interconnecting wires between bond sites such as semiconductor chip pads and/or external terminals uses ultrasonic energy to form a welded bond at each end of a conducting wire. To form such bonds, a free end of a length of bonding wire protruding from the tip of a tapered pencil-shaped bonding tool is placed in contact with a pad. The tool tip is then pressed against the wire, and energized with ultrasonic energy supplied by an ultrasonic transducer for a short time interval. The combination of a normally downwardly directed pressure applied by the tool to the contact region between the lower surface of the wire and the upper surface of the pad, combined with an oscillatory scrubbing motion at an ultrasonic frequency of the tool tip, in a horizontal direction parallel to the pad, causes an inter-molecular diffusion bond, sometimes referred to as a “weld,” to be formed between the wire and pad. The tool is then moved in an arc-shaped path to another bond site. Motion of the tool tip away from a first, “source” bond site to a second “destination” bond site causes wire supplied from a supply reel or spool to an upper entrance opening of a wire feed bore through the tool, to be withdrawn from a lower exit opening of the bore and form an arch-shaped interconnecting segment between the first and second bond sites. The tool is then moved downwardly to press a trailing portion of the wire segment against the second bond site, and the ultrasonic transducer once again energized to bond the trailing end of the wire to the second bond site. After the second or last bond in a series of bonds has been thus formed, the wire is severed at the last bond site.
In view of the very small sizes of both the micro-circuit pads and bonding wire, it can be appreciated that ultrasonic bonding of connecting wires to integrated circuit pads or similar bond sites must be performed using an apparatus such as a bonding machine which permits the tool to be manipulated to precisely controllable positions within a coordinate space which encompasses a work area containing a workpiece.
Typical wire bonding machines used for ultrasonic welding of wires to micro-circuit pads include an elongated, generally cylindrically shaped force-applying member or “tool” which has a pointed lower end. The tool is usually vertically disposed, and has a shank mechanically coupled at an upper end thereof to a source of ultrasonic energy, such as a piezoelectric transducer which is connected to an electrical energy source alternating at an ultrasonic frequency. Usually, the tool is connected to the transducer-through a tapered horn structure that matches the acoustic input impedance of the tool to the output impedance of the transducer, which typically has a larger diameter than the tool shank.
One type of ultrasonic bonding tool used to bond wires to micro-circuit pads is referred to as a wedge bonder and has a flat lower working face adapted to press a bonding wire into contact with a pad while ultrasonic energy is applied through the tool to the wire to form an ultrasonic weld. This working face is usually quite small, typically having a rectangular shape only about a few mils on a side, to permit bonding wire to small micro-circuit pads, without contacting adjacent circuit elements. The bonding process typically includes the steps of first viewing a particular workpiece pad and tool tip in a stereo microscope and video camera to align a workpiece relative to a bonding machine, and then using an automatic actuator system to position the tool tip at consecutive bond site locations on the workpiece, using a control system which employs pattern recognition logic.
In most wire bonding machines which use a wedge bonding tool, the bonding tool is so constructed as to facilitate the positioning of bonding wire over a pad, prior to performing the bonding operation. Such bonding tools typically include an oblique face which angles upwardly and rearwardly from a flat lower working face referred to as a “foot,” and have a generally vertically disposed rear side. An angled wire guide or wire feed bore having an entrance aperture in the rear side and an exit aperture in the angled lower face of the tool, rearward of the foot, is provided to enable bonding wire supplied from a reel or spool mounted upwardly and rearwardly of the tool to enter the entrance aperture of the wire feed bore, pass freely through the bore, and to be paid or drawn out through the exit aperture in the angled lower face of the tool. Typically, a remotely actuable wire clamp located rearward of the wire feed bore entrance aperture and movable with respect to the tool is used to feed bonding wire through the wire feed bore.
The wire clamp used to push wire through the wire guide bore of a bonding tool typically consists of a pair of jaws or clamp blades that may alternately be closed to grip the wire, and opened to allow free travel of the wire. Generally, such clamps may be moved toward and away from the guide bore entrance, typically on a line of movement which coincides with the axis of the guide bore. To feed wire through the guide bore, the jaws of the clamp are first opened, and the clamp then moved away from the guide bore. The jaws are then closed to grip the wire, and then moved towards the guide bore, thus feeding wire through the guide bore.
In wire bonding machines of the type described above, the machine is used to translate the bonding tool horizontally to locate the tool tip over a first bond site of a pair of bond sites, such as a pad on an integrated circuit die. The bonding tool is then moved downwardly to press a bonding wire into contact with the first bond site, and the tool is ultrasonically energized to make the first bond. The tool is then translated upwardly from the first bond site, and horizontally to a second bond site. During this motion, wire anchored at one end to the first bond site pays out wire through the guide bore exit aperture. The tool is then moved downwardly into contact with the second bond site to form a second bond. In this manner, any desired number of pads or other elements of a circuit can be interconnected together, in a procedure referred to as “stitch” bonding.
After the second or last bond in a series of bonds has been made, the wire must be severed, to permit making bonds between other pairs of bond sites. In one method of severing the wire, wire clamp blades are closed upon wire rearward of the bonding tool, and the clamp is translated rearwards from a second bond site to exert tension on the bonding wire sufficient to sever the wire. The clamp is then fed forward to feed a new length of wire from the tool. Alternatively, the wire may be severed by a “table tear” method, in which a table or platform holding a workpiece is translated forward from the tool to tension and thereby sever the wire, while the wire is held by closed clamp blades.
In moving a wedge bonding tool from a first bond site to a second bond site, the tool must be translated rearward from the first site to the second site, in a vertical plane containing both the longitudinal axis and wire-guide bore axis of the tool. This requirement results from the fact that wire paying out forwardly through the exit aperture of the bonding tool tip must remain in the plane containing the longitudinal and guide bore axes of the tool, to ensure that the wire will not bind on the exit aperture chamfer, or become twisted.
Because of the requirement for translating a wedge bonding tool from a first to subsequent bond sites in the plane of the bonding tool longitudinal axis and wire guide bore axis, many existing wedge bonding methods require that a workpiece be rotated to align a direction vector between the two sites with the bonding tool plane, and subsequent translation of the bonding tool rearwardly in that plane along the direction vector.
One method of performing the required relative translations and rotations of a wedge bonding tool relative to a workpiece utilizes a support platform for the workpiece, which is translatable in an X-Y plane perpendicular to the longitudinal axis of the bonding tool, and rotatable in the Y—Y plane. With this method, the bonding tool need only be translatable downwardly, in a minus −Z direction to effect a bond, and upwardly in a plus +Z direction after a bond has been made.
The manufacture of production quantities of microcircuits generally requires the use of automated ultrasonic bonding machines performing bonding operations of the type described above, to achieve satisfactory production rates at reasonable unit costs per circuit. An example of such automated ultrasonic bonding machines is disclosed in the present inventor's application Ser. No. 09/570,196, filed May 12, 2000, for an Automatic Ultrasonic Bonding Machine With Vertically Tiered Orthogonally Translatable Tool Support Platforms, now U.S. Pat. No. 6,382,494. The disclosed machine includes a positioning mechanism for automatically translating the tip of an ultrasonic bonding tool by drive motors to precisely pre-determinable positions within a three-dimensional coordinate space containing a workpiece. The machine described in that application also provides means for translating a bonding tool in X-Y directions parallel to a plane containing a workpiece to position the tool tip over a particular bond site, translating the tool downwardly in a minus −Z direction to make an ultrasonic wire bond, translating the tool upwardly to withdraw the tool tip from the first bond site, and translating the tool in an X-Y direction to position the tip over a subsequent intended bond site, and form thereat a subsequent bond. Thus, the disclosed machine eliminates the a requirement for a rotatable X-Y table for supporting a workpiece, and provides a highly effective method for making bonds on workpieces located on a conveyor, for example.
Operation of automated ultrasonic bonding machines typically requires an operator to select a pre-programmed operation cycle for the machine and arrange for a quantity of workpieces to be automatically presented to the machine, on a conveyor, for example. Pattern recognition logic is then used to position alignment targets of a workpiece precisely with respect to an ultrasonic bonding tool, whereupon a sequence of automatic bonding operations joining individual interconnecting wires between a plurality of first and second bond sites may be initiated. Each work piece may require bonding dozens or even hundreds of individual interconnecting wires between separate pairs of bond sites.
In ultrasonically bonding an interconnecting wire between a pair of first and second bond sites, a length of wire which enters an entrance opening of a wire feed bore through the tool tip from a supply reel or spool and which protrudes from an exit opening of the bore located rearwards of a front foot of the tool is pressed down between the foot of the tool at a first bonding site such as a conductive pad on an integrated circuit chip. Ultrasonic energy is then applied to the tool, causing the tip of the tool to vibrate fore and aft at an ultrasonic frequency. Scrubbing action of the lower surface of the wire against the pad causes the contacting surfaces between the pad and wire to be ground together. This scrubbing action in turn results in the plastic deformation of microscopic surface protrusions, pushing the peaks of the protrusions into valleys of a contacting surface, dispersing impurities from the surfaces and bringing nascent molecules of the wire and pad so close together that they intermingle and form a solid-state diffusion bond. Even though such bonds are formed at room temperature, they are sometimes referred to as welds. The bond or weld secures the wire to the pad mechanically and in electrically conductive contact with each other. The tool tip is moved upwards from the first bond site, and translated rearward from the first bond site to position the tool tip foot over a second bond site, such as a terminal pad. The bonding tool is again moved downwards to press the length of wire against the second conductive pad, and ultrasonic energy once again applied to the tool tip to effect a second bond. Upward motion of the tool tip in conjunction with lateral motion of the tool tip between a pair of bond sites causes wire paid out from the tool between the sites to form an arch-shaped arc, which is longer than the point-to-point distance between bond sites. The arch-shaped length of interconnecting wire between bond sites is provided to avoid electrical contact between the interconnecting wire segment and other portions of a work piece.
After a pair of bonds has been made as described above, a continuous length of bonding wire interconnects the first and second bond sites, and trails rearwardly away from the second bond site under the tool. The trailing portion of wire must be severed by either of two methods, to finish the interconnection and prepare the bonding tool to make another pair of first and second bonds. According to one method of severing a wire at a second bond site, jaws of a wire clamp located behind the tool are closed to grip the wire between the tool and a wire supply reel, and the clamp moved rearward to exert tension on the wire sufficient to break the wire. According to another method of severing a wire at a second bond site, sometimes referred to as a “table tear,” a clamp secures the length of wire rearward of the tool, and a platform or table to which a workpiece is secured is translated forward from the tool. In both methods, tension exerted on wire causes it to break at the rear area or heel of the second bond.
After a length of bonding wire from a supply reel has been severed at a second bond site by either of the two methods described above, the wire clamp rearward of the tool is moved forward to feed wire outwardly through the bonding tool bore a distance sufficient to underlie the front foot area of the bonding tool, thus positioning the wire correctly for making a first, source bond at a different bond site.
A problem which can occur using automatic ultrasonic bonding of the type described above is failure of a length of bonding wire to bond to a site. Accordingly, it would be desirable to provide means for detecting failure of a bonding wire to properly bond to a bond site at the time the failure occurs so that a bonding sequence may be halted and corrective action taken.